The last six decades of Middle Eastern history can be neatly divided into three phases: The first began with Gamal Abdel Nasser's 1952 revolution in Egypt, the second with the Arab world's humiliating loss in the 1967 war with Israel and the third with the 1979 Islamic revolution in Iran.

A fourth phase likely started over the weekend in Egypt. But whether the political "reform" movement in Cairo's streets turns out to be a positive or negative turn for the region—and for the U.S.—depends much on Hosni Mubarak, Mohamed ElBaradei and, to a lesser extent, Barack Obama. If history is any guide, it may take months, if not years, to know precisely the outcome.